Due to the potential risk of infection, contact lenses are required to be supplied to the end user in a sterile state. The level of sterility required is governed by various U.S. Food and Drug Administration (FDA) guidelines and also European Standard EN 556. Both state that the theoretical probability of there being a viable micro-organism present on/in the lens must be equal to or less than 1×10−6. This is often expressed as a Sterility Assurance Level (SAL) of 10−6 or 6 log.
This required level of sterilization is usually achieved by terminal sterilization, meaning that the lenses are sterilized at the end of the manufacturing and packaging process. The sterilization process typically involves some form of temperature and/or pressure-based sterilization technique. For example, the lens and a quantity of storage saline are sealed in a final shipping package and then subjected to a terminal sterilization process that typically involves heating the package in an autoclave to a temperature that insures sterilization.
Specifically, the packaged lens is sterilized by placing the package in an autoclave at an elevated humidity, temperature and pressure for an extended period of time, usually at least 15 minutes, and more typically 30 minutes, at 121° C. at a pressure of 1 atmosphere. In the case of lenses packaged in blister packs (the accepted method of packaging disposable contact lenses), there is the additional requirement to balance the pressure changes during heat-up and cool down to prevent the blister packages from bursting. This required balancing has the effect of prolonging the autoclave cycle.
Although this commercial process produces thoroughly sterilized contact lenses, the batch-wise autoclave sterilization step is time consuming, costly, and inefficient. It also detracts from the otherwise flow-line manufacturing process required to economically manufacture high volumes of disposable contact lenses, particularly for a daily-wear, disposable modality.
Amongst other negative effects of the autoclave process are the potential effects of the high temperature on the lens packaging and package contents. For instance, contact lens blister packages are generally fabricated from an injection molded polypropylene to form a boat that is closed with a laminated aluminum foil. Whilst polypropylene is relatively immune to the effects of the temperatures typically experienced in autoclave processing, some mechanical distortion may occur. This distortion is usually avoided by forming a relatively thick-walled boat or blister. While this generally prevents the distortion, it means that thin-walled packaging cannot be used even though the thin-walled packaging would be less-expensive and easier to work with.
Similarly, the temperature and pressure changes experienced during the autoclave process may cause some cosmetic deterioration of the foil used to close the boat. This often results in a slightly wrinkled foil. The effects of water applied at high temperature and pressure within the autoclave will also limit the use of pre-printed foils to inks compatible with the autoclave process. Currently, this limitation is generally overcome by means of an overlabel, which is applied to the lens pack after autoclaving, and hence adds another step to the manufacturing cycle.
The requirement for autoclaving may also complicate the use of certain saline additives, such as hyaluronic acid, due to hydrolysis at elevated temperatures. In the case of hyaluronic acid, hydrolysis during autoclaving resulting in an undesirable lowering of the mean molecular weight of the hyaluronic acid, along with an increase in its polydispersity.
The use of autoclaving also requires complex pressure equipment, and each autoclave load requires careful monitoring of the temperatures throughout the chamber. In the event of a failure to demonstrate that the required temperature within the chamber was held for the prescribed time to satisfy legal sterilization requirements or guidelines, the lenses will require re-autoclaving or scrapping. Such failures may occur due to a fault in the actual autoclave itself, or, more commonly, due to a failure of a temperature sensor within the autoclave.
Efforts have been made to avoid these disadvantages of the typical autoclave based terminal sterilization process. U.S. Pat. No. 4,464,336, for instance, teaches a method of sterilization utilizing an ultra-violet flash discharge to produce an intense pulse of UV light. Similarly, U.S. Pat. Nos. 5,034,235 and 4,871,559 disclose the use of intermittent, short-duration pulses of very intense light, containing both visible and ultra-violet frequencies to inactivate microorganisms on the surfaces of food products. U.S. Pat. No. 5,786,598 and U.S. Pat. No. 6,592,816 teach the application of this technology to the sterilization of contact lenses.
Whilst the teachings of U.S. Pat Nos. 5,786,598 and 6,592,816 would allow for a flow-line manufacturing process, there are some important limitations with this approach. Firstly, if the contact lens contains a UV blocker, sufficient absorption of the incident ultraviolet light may occur so as to preclude the inactivation of microorganisms. Secondly, any method reliant upon irradiation of the lens and lens package contents will be less effective or entirely ineffective for a non-transparent package, such as disclosed in U.S. Patent Application Publication No. 200402383801.